A power amplifier (PA) circuit may be biased for different modes, or “classes” of operation. Exemplary classes include Class A, Class AB, and Class B. In Class A operation, a PA may be biased such that the PA is in a conducting, or ON, state during 100% of the cycle, or the entire cycle, of the input signal. The bias level is also typically selected such that the PA operates in the most linear portion of the transfer curve, which characterizes the PA circuit. In Class A operation, the output signal from the PA is typically a scaled version of the input signal, where the scaling factor is a function of the gain associated with the PA circuit. However, because of the bias level utilized for Class A operation, the PA is typically in a conducting state even when there is no input signal. Furthermore, even when the PA is amplifying an input signal, the efficiency of the PA may not exceed 50%. For example, each watt of delivered output power, or Pout, may require two (2) watts of delivered power, PDC, from a DC power supply source (such as a battery). One limitation of Class A PA circuits for use in mobile wireless communication systems like wireless local area network (WLAN) systems is that high bias levels often utilized to enable large variations in output power levels may result in unacceptably short battery life and/or high levels of generated thermal heat.
In Class B operation, a PA may be biased such that the PA is in a conducting state during 50%, or half, of the cycle of the input signal. This may result in large amounts of distortion of the input signal in the output signal. In this regard, in Class B operation, the PA may operate in a nonlinear portion of the transfer curve. However, the theoretical efficiency of a Class B PA circuit may reach 78.5%. The higher efficiency of the Class B PA results from the PA being in a non-conducting, or OFF, state half of the time. While the PA is in the OFF state, power dissipation may be theoretically zero (0). One limitation of Class B PA circuits is that distortion levels in output signals may be unacceptably high.
In Class AB operation, a PA may be biased such that the PA is in a conducting state for greater than 50%, but less than 100%, of the cycle of the input signal. In Class AB operation, the PA may be more efficient than in Class A operation, but less efficient than in Class B operation. Furthermore, in Class AB operation, the PA may produce more distortion than in Class A operation, but less than in Class B operation.
A power amplification circuit in a wireless system is typically a large signal device. In WLAN systems, the power amplifier circuit may transmit output signals at average power levels in the range of 10 dBm to 15 dBm, and peak power levels of about 25 dBm, for example. In WLAN systems, which use OFDM or CCK modulation, output power levels may vary widely such that the ratio of the peak power level to the average power level may be large, for example, 12 dB for OFDM and 6 dB for CCK. Because of these large swings in output power levels, power amplifier (PA) circuits may distort the output signal. Distortion, however, is a characteristic, which may be observed in PA circuits that are utilized across a wide range of applications, and may not be limited to PA circuits utilized in wireless systems. There are two metrics, which may be utilized to evaluate the distortion performance of PA circuits. These metrics may be referred to as amplitude modulation to amplitude modulation (AM-AM) distortion, and amplitude modulation to phase modulation (AM-PM) distortion.
The AM-AM distortion provides a measure of the output power level, pout, in response to the input power level, pin. The input power level, and output power level are each typically measured in units of dBm, for example. In an ideal, non-distorting, PA circuit, the output power level changes linearly in response to a change in the input power level. Thus, for each Δpin change in the input power level there may be a corresponding change in the output power level, Δpout≈Δpin. The AM-AM distortion may be observed when, for example, the output power level in response to a first input power level may be pout1≈αpin1, where the output level in response to a second input power level may be pout2≈βpin2, when α≠β.
The AM-PM distortion provides a measure of the phase of the output signal in relation to the input signal (or output phase) in response to the input power level. Output phase is typically measured in units of angular degrees. The AM-PM distortion may be observed when, for example, the output phase changes in response to a change in input power level.
When the peak input signal level to a PA circuit is large compared to the average input signal level, or high peak to average ratio, the PA circuit may be biased to accommodate the peak input signal level, PINMAX. The value of PDC may be set to enable generation of an RF signal output level from the PA circuit, PRFMAX, when the corresponding input signal level is PINMAX. Thus, efficiency of the PA circuit may be highest for a given value PDC when the RF signal output level from the PA circuit is PRFMAX. However, for high peak to average ratios, the input signal level is typically less than PINMAX for a substantial portion of the time that the PA circuit is operating. Therefore, the average RF signal output level, PRFAVG, may be significantly lower than PRFMAX. Consequently, the need to support high peak to average ratios may result in low efficiency for the PA circuit.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.